The following will explain a common method for producing a TCI (time-compressed integration) signal from a wide-band video signal. With this method, assuming that a luminance signal Y whose bandwidth is 20 MHz and color-difference signals P.sub.R, P.sub.B having a bandwidth of 6 MHz are used as baseband HDTV signals, the signals of three channels with different transmission bandwidths are transmitted over one or two common channel(s) by time-division multiplexing and a TCI signal is composited.
FIG. 5 illustrates a system for encoding/decoding such baseband signals into a two-channel TCI signal, and the waveforms of input and output signals of a TCI encoder 51 are shown as an example in FIG. 6. A TCI signal T.sub.1 of channel 1 is produced through the processes shown in FIG. 6. As for a luminance signal Y.sub.1 for an odd numbered line, the duration of one entire scanning line is 29.63 .mu.s and the bandwidth is 20 MHz. The TCI encoder 51 firstly converts a portion of the luminance signal Y.sub.1 corresponding to an effective scanning period into Y.sub.1T with a bandwidth of about 12 MHz by time-expanding it by 1.68 times. Regarding a color-difference signal P.sub.R1 of the 6-MHz bandwidth for the odd numbered line, the TCI encoder 51 converts a portion of the color-difference signal P.sub.R1 corresponding to the effective scanning period into (PR.sub.1).sub.T with a bandwidth of about 12 MHz by time-compressing it to half of its original length. Then, the TCI signal (T.sub.1) is produced by compositing Y.sub.1T and (P.sub.R1).sub.T through the time-division integration operation. Next, a TCI signal (T.sub.2) of channel 2 is produced from a luminance signal Y.sub.2 and a color-difference signal P.sub.B2 for an even numbered line in a similar manner by the time-division integration operation.
A line-sequenced color-difference two-channel TCI signal having the 12-MHz bandwidth is thus obtained by time-expanding the luminance signal Y and the color-difference signal P.sub.R or P.sub.B, extracted from every other line, and by performing time-division integration. In the time-division integration of the luminance signal Y of a bandwidth of 20 MHz and the color-difference signals P.sub.R, P.sub.B of the 6-MHz bandwidth, the bandwidth is calculated by (20+6) MHz/2=13 MHz. However, with the time-division integration, only the information corresponding to the effective scanning period is used, and therefore a bandwidth of 12 MHz is achieved.
The TCI signals of two channels are recorded in parallel on a magnetic tape. The TCI signals reproduced from the magnetic tape are converted into the original baseband signals by a TCI decoder 52. In the TCI decoder 52, the reverse processes of the TCI encoder 51 are performed. In the reproduced signals, since the P.sub.R or P.sub.B signal is missing every other line, the baseband signals are reconstructed based on line interpolation.
To record the TCI signal of the 12-MHz bandwidth with VTR, an FM signal obtained by frequency-modulating a 19-MHz carrier frequency with the TCI signal is recorded on the magnetic tape. The spectrum of the FM signal is shown in FIG. 7(a). It can be seen from the figure that the lower edge of the lower sideband is 7 MHz. In reality, however, there exist a relatively large number of components whose frequencies are below this sideband.
The following explains the case where a pulse-count-type FM demodulator (wherein a one-shot multivibrator is actuated at every zero crossing point of the FM signal) is employed and the reproduced signal does not contain the even higher harmonics of the carrier. In this case, the demodulated signal consists of components, namely, the TCI signal, harmonics having a frequency that is twice the FM signal frequency, odd harmonics of the frequency-doubled harmonics. In theory, the fundamental waves of the FM signal are not produced. The second harmonics have a center frequency of 38 MHz, and the lower edge of the lower sideband is 14 MHz. It is thus possible to separate TCI signal of the 12-MHz bandwidth from the demodulated signal. However, in fact, the fundamental waves of the FM signal leak due to various reasons, and thereby the DU (Desired signal to Undesired Signal) ratio of the TCI signal results in around 30 dB to 40 dB.
Then, the following method using a frequency doubler is proposed. In this method, as illustrated in FIG. 7(b), the reproduced signal is first input into the frequency doubler where its center frequency is raised to 38 MHz, and then input into a pulse-count-type FM demodulator where the frequency is doubled. As a result, the lower edge of the lower sideband becomes 28 MHz as indicated in FIG. 7(c), making it easy to separate the TCI signal. However, if the components of the FM signal leak, the leaked components are contained in the output signal of the FM demodulator. Moreover, if the output of the frequency doubler contains an FM signal whose frequency is four times that of the input signal as well as the FM signal with a frequency twice that of the input signal, the FM signal with the frequency twice that of the input signal is contained in the output signal of the demodulator as shown by the alternate long and one short dash line in FIG. 7(c). When these lower sideband components are mixed in the TCI signal, interference beats are produced.
Meanwhile, according to an FM carrier reset method wherein the phase of an FM carrier (hereinafter an FM signal is referred to as an FM carrier) corresponding to the front porch of a horizontal sync pulse is taken as a reference phase and the phase of an FM carrier corresponding to the tip portion of the horizontal sync pulse is reset to the reference phase every line (see Japanese Publication for Unexamined Patent No. 274290/1988). This method brings about a horizontal correlation between the FM carriers. Consequently, the correlation between the lower sideband components and the reproduced image becomes significant. This stops the running of beat stripes which appear in the reproduced image on the screen when the lower sideband components of the FM carrier leak into the demodulated signal. Therefore, although distortion of the image occurs, deterioration of quality of the image can not be detected as the distortion is almost invisible.
In order to fully enjoy the horizontal correlation effect, the applicant of the present invention proposed a method of recording or reproducing an FM carrier which has shifted to a lower frequency (see Japanese Publication for Unexamined Patent No. 48391/1991). In this method, as illustrated in FIG. 8(a), while the parameters of frequency modulation (TCI signal bandwidth: 12 MHz, emphasis level: 12 dB at 11 MHz) are unchanged, the center frequency shifts from 19 MHz to 15.5 MHz. In this case, the reproduced signal is doubled by the frequency doubler as shown in FIG. 8(b) and then multiplied by four times by a pulse-count-type FM demodulator as indicated in FIG. 8(c). The lower edge of the lower sideband of the FM carrier is 14 MHz. Since the center frequency is close to the TCI signal band (the video signal band), some sideband components may leak and cause detectable image distortion. Therefore, as a strictest test, recording and reproduction of a 100% multiburst signal was performed. The results are that, even when image distortion caused by the multiburst signal is in a undetectable level, distortion of waveform reaches around 10%.
In order to minimize signal distortion to a invisible level on the screen, the signal must be recorded on a magnetic tape with the modulation method in which the phase of an FM carrier corresponding to the tip portion of a horizontal sync pulse is reset to a reference phase. An example of the operation with this method is briefly explained below.
As shown in FIG. 9, a horizontal/vertical pulse separation circuit 61 and a master clock generator 62 are interlocked such that the master clock generator 62 produces pulses which are phase-locked with horizontal and vertical sync pulses separated from the TCI signal by the horizontal/vertical pulse separation circuit 61. In this case, if the duration of a single scanning line of the TCI signal is equal to that of two scanning lines of the luminance signal Y (see FIG. 6), it is possible to simplify the synchronizing panel of the TCI encoder. Further, this arrangement enables the unification of clock systems used for processing signals in a video tape recorder.
After the high frequency region of the TCI signal is emphasized in a pre-emphasis circuit 63, the TCI signal is input into a multivibrator 64 for frequency modulation. The multivibrator 64 is connected to a pulse generator 65, a pulse generator 66, and a reference frequency generator 67 for automatic frequency control. The pulse generator 65 generates pulses for resetting the phase of a carrier corresponding to the front porch of the TCI signal to a reference phase. Meanwhile, the pulse generator 66 generates pulses for resetting the phase of a carrier corresponding to the tip portion of a horizontal sync pulse to a reference phase. As illustrated in FIG. 10, after resetting the phase of the FM carrier corresponding to the front porch of the TCI signal to the reference phase, the phase of the FM carrier corresponding to the tip portion of the horizontal sync pulse is reset to the reference phase. In general, resetting causes discontinuous variations in the phase of the FM carrier. Therefore, when these portions are frequency-demodulated, transient distortion occurs. To prevent such distortion from occurring in the horizontal sync pulses, the phase of the FM carrier corresponding to the front porch is first reset so that transient distortion occurs at the front porch. Then, when the phase of the FM carrier corresponding to the tip portion of a horizontal sync pulse is reset, discontinuous variations in the phase of the FM carrier do not occur. Consequently resetting does not cause transient distortion at the tip portion during FM demodulation (see Japanese Patent Application for Unexamined Patent No. 17980/1991). After the resetting, as illustrated in FIG. 9, the FM carrier is frequency-converted by an analog multiplier 69. A signal (48.6 MHz) sent from a local oscillator 68 to the analog multiplier 69 is in synchronous with a master clock. The output signal of the analog multiplier 69 is input into a low-pass filter 70 where the FM carrier whose center frequency has shifted to 15.5 MHz while maintaining a frequency deviation .DELTA.f at 2.6 MHz is separated. Here, the center frequency f.sub.o of the FM carrier transmitted from the multivibrator 64 is set at 64.1 MHz. The reason for setting the frequency at 64.1 MHz is to facilitate the removal of third harmonics which are produced as the output waveform of the multivibrator 64 is a square wave.
The following explains the case where a conventional demodulation method of multiplying the frequency by four times is adopted and the frequency of the carrier is lowered so as to achieve high-density recording. In this case, however, in terms of the output signal of the FM demodulator, as illustrated in FIG. 8(c), since the frequency spacing between the lower sideband of the FM carrier whose frequency is four times the input frequency and the demodulated signal band is small, even when a low-pass filter is used to remove the lower sideband components, some components are mixed in the video signal. Moreover, if recording is performed without resetting the phase of the FM carrier to the reference phase, running beat strips appear, deteriorating the quality of the image.
On the other hand, if recording is performed by resetting the phase of the FM carrier to the reference phase, running beat strips do not appear. However, the lower sideband components contained in the demodulated video signal cause distortion of image. In the case of recording and reproduction of the 100% multiburst signals, although distortion of image can not be detected, waveform distortion reaches about 10%. In other words, while moire does not appear, waveform distortion prevents reproduction of a high-quality image.